Journal of Medical Physics (Dec 2024)

Backside Irradiation of Ultraviolet-A for Correcting Nonuniformity Error of Gafchromic XR-QA2 Films

  • Nobuyoshi Tanki,
  • Sachiko Goto,
  • Toshizo Katsuda,
  • Rumi Gotanda,
  • Tatsuhiro Gotanda,
  • Tadao Kuwano

DOI
https://doi.org/10.4103/jmp.jmp_87_24
Journal volume & issue
Vol. 49, no. 4
pp. 563 – 567

Abstract

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Purpose: Radiochromic film is used for quality assurance and quality control of X-ray equipment in the diagnostic radiology. In addition, three-dimensional dose distribution of computed tomography (CT) is measured. To correct the nonuniformity and uncertainty of radiochromic films for dose measurement of CT, the films are preirradiated ultraviolet (UV)-A rays. There is a difference in the UV protection strength of radiochromic films. A concern exists about the effects of the UV-A irradiation intensity. We thus irradiated with UV-A rays from the backsides of the films to assess if backside irradiation was possible. Materials and Methods: Gafchromic XR-QA2 and RTQA2 were used in this study. The UV-A rays were simultaneously irradiated on the front and backsides of each film for 12 h. The yellow layer of each film was scanned and imaged. The average pixel values ± standard deviations (SDs) were compared. In the statistical analysis, a paired t-test was performed. To compare, the active-layer densities engendered by the UV-A rays. Calibration curve was created with 48 h of preirradiation of UV-A. Results: The mean pixel values ± SD for Gafchromic XR-QA2 on the front and backsides were 130.776 ± 0.812 and 81.015 ± 1.128, respectively. On the other hand, the mean pixel values ± SD for Gafchromic RTQA2 on the front and backsides were 62.299 ± 1.077 and 133.761 ± 1.365, respectively. The statistical results of the paired t-test were significantly different (P < 0.01) between both films. Fitting equation of the calibration curve is shown below. y = -390.47 ± 200 + (443.45 ± 10x80).5068 ± 0.0434. Conclusion: Based on the relationship between the sensitivity of the active layer to UV-A rays and the strength of UV protection on the surface, we concluded that backside irradiation is recommended for Gafchromic XR-QA2, and frontside irradiation is recommended for Gafchromic RTQA2.

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